The Northwest China region is abundant in shallow buried coal fields with thick coal seams and high coal quality, but is in an arid or semi-arid continental climate region where the water resources are in short, the vegetation coverage is low, and the ecological environment is weak. Years of mining practice has shown that: if large-scale mechanized mining is carried out in the conventional manner, large-area and severe loss of water and soil resources will occur in the mining area owing to the development of mining-induced fissures, causing a series of environmental and geological effects in the mining area and further accelerating the degradation of the ecological environment that is already very weak.
At present, existing water-preserved mining method for controlling fissures of overlying strata and surface subsidence in coal mines mainly include room and pillar mining, strip mining, grouting separated strata, and fill mining methods, etc. The strip mining method and the room and pillar mining method realizes surface subsidence control at the cost of permanent coal pillars left behind, usually achieve a recovery ratio at about 50%, and involves severe resource waste. The method of grouting separated strata mitigates surface subsidence by filling the separated spaces of the overlying strata via hole drilling and grouting way, is only applicable to situation where the overlying strata have hard upper strata and soft lower strata, and usually can only attain a surface subsidence reduction ratio not higher than 40%. The fill mining method utilizes a filling body to replace the coal, and is the most effective method for controlling surface subsidence, among which paste filling is one of the most ideal methods.
Paste filling is to process gangue, coal ash, industrial slag, and urban solid wastes nearby the coal mine into cementing or non-cementing pasty grout on the ground, transport the grout through a pipeline by means of a filling pump or under gravity to the underground area, and fill the goaf at the stope working face partially or fully, to form a necessary overlying strata support system mainly consisting of a pasty filling body, so as to effectively control fissures of overlying strata and surface subsidence and realize water resource preserved mining in the coal mine.
For paste filling, the filling body mainly functions to support the roof However, if the filling body does not contact the roof after it cures, it will lose the active roof supporting function within a crucial time period; what's more, it will aggravate surface subsidence and cause disasters. Therefore, ensuring roof-contact in the goaf is the key. However, on one hand, at present, whether the paste filling in the goaf reaches a roof-contacted state is mainly judged by observing grout overflow from the air exhaust pipeline at the peak elevation in the goaf. But, grout overflow from the air exhaust pipeline only indicates the goaf is fully filled at the moment, and can not fully manifest a roof-contact state has been attained in the goaf, because the volume of the filling body may be reduced after the filling body cures, if the mixing ratio of the filling material is not scientific or grout leakage resulted from the development of fissures in the surrounding strata in the stope occurs; such phenomena can not be monitored by conventional methods; on the other hand, in the past, no effective remedial measures were taken when non-roof-contact phenomena are found in the goaf. This is adverse for control of surface subsidence.